The present invention generally relates to the field of medical X-ray machines and more particularly relates to an actual skin input dose rate computing device and method and an X-ray machine.
At present, X-ray machines have been increasingly used in medical field. An X-ray machine images a subject by emitting X-ray through the subject. The doctor diagnoses and treats the subject based on the resulting image. However, as is well known, X-ray is harmful to the human body. Therefore, the X-ray dose rate incident on the subject skin needs to be known, such that the exposure time can be effectively controlled in order to minimize the harm caused to the subject by X-ray.
In Germany and some other countries in Europe, X-ray machines used in hospitals, e.g. C-arm X-ray machines, are required to provide DAP (Dose Area Product) to indicate the dose applied to the subject.
In the United States, the latest 21CFR (Title 21 of the Code of Federal Regulations) 1020.32(K) requires that the values of AKR (Air Kerma Rate) and CAR (Cumulative Air Kerma) at a given reference position should be displayed. For fluoroscopic devices produced on and after Jun. 10, 2006, AKR and CAR should be displayed at the operating position of the user.
In 21CFR 1020.32 (K), it is pointed out that for an X-ray system, the reference position is fixed, i.e. the position 30 cm above the upper surface of an X-ray detector.
According to the above requirement, a doctor could know a total absorbed dose or dose rate at a given reference position. However, the doctor usually cannot place the subject right at the reference position, but instead places the subject at the best diagnostic position. Thus, the doctor cannot know the actual skin input dose. The actual skin input dose is different from the input dose at the reference position. Sometimes actual skin input dose rate might be three times higher than the input dose rate at the reference position or even more. Therefore, even if the skin dose rate at the reference position is very low, it could still hurt the skin of the subject, because the actual skin input dose rate might be three times higher than the skin dose rate at the reference position or more, or even up to tens of times higher.
U.S. Pat. No. 6,330,299 discloses a system and method for determining the dose area product in an X-ray imaging system, which computes DAP based on a set of X-ray specification and image specification and then obtains the skin input dose rate RR at the reference position (as defined in 21CFR 1020.32 (K)).
U.S. Pat. No. 6,934,362 discloses an X-ray system and method for determining an effective skin input dose in an X-ray examination. In this patent, the skin input dose is obtained by dividing the measured DAP by exposed skin input area, which is calculated from the exposed area in the film or image intensifier plane.
U.S. Patent Publication No. 2005/0152498 discloses a method for computing the ray dose rate at a given position, which creates a table during the machine design and looks up the ray dose rate in the table based on the kilovolt and current values of the exposure parameters.
The technical solutions in the disclosed patents and patent applications mentioned above can only give the input dose rate at a specified reference position, but none of them can give the actual skin input dose rate.
There is a method for measuring the actual skin input dose rate at present, which measures the skin input dose rate in real time by placing a dose rate testing instrument on the skin surface of the subject. However, due to the placement of the dose rate testing instrument on the skin surface, the dose rate testing instrument will appear in the X-ray images, thereby impeding the doctor diagnosing diseases. This is highly infeasible in a practical surgery.